The storm surge flood warning system was implemented under the Plan to Prevent Coastal Flooding and Flash Floods, presented by Jean-Louis Borloo, Minister of Ecology, Energy, Sustainable Development and the Sea, at the Council of Ministers on 13 July 2010.

What is storm surge flooding?

Storm surges can cause severe flash flooding along the coast, in harbours and the mouths of rivers.

They are caused by extreme high sea levels due to the combination of several events:

the intensity of the tide (sea level mainly due to astronomical phenomena and geographical configuration). The higher the tidal coefficient, the higher the high tide level.

the passage of a storm, producing a rise in sea level (called storm surge) through three main processes:

strong swell and waves cause the sea level to rise;

the wind exerts friction on the surface of the water, which changes the currents and sea level (accumulation of water near the coast);

a decrease in the atmospheric pressure. The weight of the air decreases at the surface of the sea, and naturally, the sea level rises. A decrease in atmospheric pressure of one hectopascal (hPa) is equivalent to an increase of approximately one centimetre of water level. Example: A low pressure system of 980 hPa (a difference of 35 hPa relative to the average atmospheric pressure of 1015 hPa) generates a rise of about 35 cm.

Breaking waves result in a movement of water masses propagating along the foreshore (zone covered and uncovered at each tide). Jetties, seawalls and other coastal structures can be overrun, damaged or weakened.

Aggravating factors

If the events described above occur simultaneously, this aggravates the flooding and allows the sea to breach sea walls and flow into areas that are usually sheltered. The severity of the flooding depends on the water level reached, incoming water volume and how fast the water flows back out. The intensity of these events depends strongly on the configuration of the seabed, the foreshore and the geographical features of the coast, such as:

shallower sea (upon arriving on the coast, the wave energy is transformed into rising water levels);

the nature of the seabed, which slows or accelerates the propagation of the wave towards the coast (sand, gravel, mud)

the orientation of the coast with respect to the direction of wave propagation.

Two recent examples

During the passage of the storm Xynthia (27-28 February 2010), the sea level rose in places to over 2 meters in homes. That night, the weather conditions caused the sea level to rise (storm surge) by 1.53 m in La Rochelle, at a time when the sea level was at its highest (high tide with a coefficient of 102 and strong swell). The sea rose more than one meter above the level of the highest tides ever observed.

On January 1, 2010, the Côte d'Azur and Corsica were affected by wave trains exceptional for the region. The Nice buoy recorded significant heights of 4 m. These waves from the Balearic Islands, associated with a storm surge of more than 50 cm, caused major flooding on the coast, from Hyères Islands to Monaco and on the west coast of Corsica.

Dangers

Strong waves and coastal flooding are destructive. They can affect the entire coast of Metropolitan France, including on the Mediterranean, where the tidal amplitude is low.

The flooding primarily affect low lying areas near the coast. Storm surges, however, can can produce coastal flooding that reaches several kilometres inland, with water levels of several meters. Channels of communication, housing and businesses can be flooded and damaged in a few hours or less.

Waves can damage coastal infrastructure (seawalls, jetties, etc.) and carry objects or materials that can injure people, damage property and impede traffic along the coast.

Items not properly secured may be washed away by flood waters.

Boats moored in harbours can be washed ashore.

Near estuaries, the flow of rivers can also be slowed or stopped, which then generates flooding.

The damage can be exacerbated in the event of gusty winds, heavy rain and broken levees.

Injuries and material damage caused by waves and flooding depend on natural factors but also on human activities (land use). Damage can be reduced through protective measures (seawalls, jetties, dunes) and preventive measures (restrictions on developments in exposed areas, information, preparedness).